The present invention relates to a device for indexing the arm supporting a rearview mirror casing, or for indexing the casing of a vehicle rearview-mirror. It is known to use devices for holding the arm supporting a rearview-mirror casing, or the rearview mirror casing proper, in a predetermined position with respect to a supporting member fitted on the vehicle. These devices are also designed to allow the rearview-mirror casing to move in the event of impacts. These currently known indexing devices, however, are often complex and are composed of a rather large number of parts which makes them rather difficult to assemble and expensive to produce.